Electric lamp



Nov. 14, 1950 H. R. LEVY ELECTRIC LAMP Filed J u1y 11, 1944 V.. ,w R mL F L R v0 o Nw w 1R A m Patented Nov. 14, 1950 ELECTRIC LAMP Hana Rudolf Levy, Golden Green, London, England, assigner to Thom Electrical Industries Limited, London, England, a company of Great Britain Application July 11, 1944, Serial No.544,369 In Great Britain July 29, 1943 19 Claims. l1

This invention relates to means for illumination comprising an electric discharge lamp of the kind having a luminescent surface coniined within an evacuated envelope and adapted to be excited to luminescence predominantly by impact of electrons liberated into the discharge space of the lamp. The envelope may be highly evacuated, or it may contain traces of gas. For brevity., such a lamp is hereinafter referred to as a cathodo luminescent lamp.

An object of this invention is to provide such a lamp having a cathode not requiring any heating before or during operation.

Another object is to provide a luminescent discharge lamp which can be started without a special starting switch and without any appreciable delay at a low voltage such as of the order of several hundred volts, or in some instances at mains supply voltage, the voltage not depending on the length of the lamp envelope.

A further object is to provide such a lamp having a high efficiency of conversion of electrical into light energy and at the same time a satisfactory life.

A further object is to render such a lamp capable of operation by alternating current without an objectionable degree of flicker.

A further object is to enable the variety of luminescent materials suitable for use in such a lamp to be extended.

A further object is the provision of a device for illuminating purposes comprising in combination a cathodo luminescent lamp and a source of light of a different character, the device being adapted to provide conversion of at least part of the visible or invisible light of said light source into luminescent light having a different wavelength range and, in some instances, with an improvement of the energy conversion efciency of said light source.

Further objects and advantages of the invention will appear from the following description of certain embodiments of the invention shown diagrammatically and by Way of example in the accompanying drawings, in which:

Fig. l is a longitudinal sectional view of a luminescent discharge lamp according to the inventlon;

Fig. 2 is a cross section of the lamp of Fig. l taken along the line 2 2;

Figs. 3-5 are fragmentary cross-sectional views of modifications of my invention.

Referring to Figs. 1 and 2, the lamp comprises a glass envelope 2 which has double walls 2a, 2b,

the walls being approximately parallel to each other, and the substantially annular` space between the walls being narrow in relation to their circumference and length, similar to that in a Dewar vacuum flask; the envelope is thus a double-walled ring or, as in this example, a

double-walled cylinder of some length. On a separate, translucent support or, preferably on the curved interior surface of the outer glass wall 2a of the lamp envelope 2, the luminescent material 3, for example a zinc orthosilicate, is arranged as a translucent layer according to known principles. The layer 3 is rendered conductive in f any suitable manner, for example by a translucent conductive backing (not shown) which may be a thin silver deposit. On the opposite side, namely the curved interior surface of the inner wall 2b of the envelope, a. translucent photoemissive material 4 is provided. vIt may consist of a thin translucent nickel or copper layer which is activated by depositing on its surface of an oxidised silver layer and an alkaline metal, such as caesium, rubidium, lithium, potassium, or sodium.

The conductive luminescent layer 3 is connected to the positive pole of a suitable source of potential, the negative pole of which is connected to the photoemissive cathode 4, thus impressing an electrostatic field between the photoemissive layer 4 and the luminescent layer 3.

An auxiliary electrode, such as a perforated plate, or a wire mesh grid, may be placed between said photoemissive layer 4 and the luminescent layer 3. The auxiliary electrode may be arranged to surround substantially the whole of the photoemissive layer 4, and may be biased to a more positive potential.

A light source 5 is positioned within the open, hollow space surrounded by the double-Walled ring or cylinder 2. When this cylinder is of substantial length, the light source 5 may consist of a tubular incandescent lamp (as indicated in Fig.

v1). The envelope6 of the incandescent lamp,

or other light source, may be made light-diffusig so that its light emission is approximately equal in a direction towards the surrounding lamp envelope 2. Alternatively, the auxiliary light source 5 may be so placed, and its light emission so directed-that the light falls into the space between the photo-emissive cathode 4 and the luminescent layer 3` In operation, the light emissi n from the light source 5 causes a release of ph/gtolectrdns from the photoemissive cathode 4 within the lamp 'envelope 2. The liberated photoelectrons are accelerated towards the luminescent layer 3 under influence of the electric field set up between the conductive luminophore 3 (and the auxiliary electrode, if any) and the photoemissive cathode 4, and excite the luminophoric layer 3 to emit light. The luminescent light emitted from the layer 3,

c under the impact of the photoelectrons, is partly radiated to the outside and partly emitted towards the interior of the lamp, namely towards the light-responsive cathode 4 which is close to the luminophore 3 and therefore fully exposed 3 to its light radiation. The light so emitted from the layer 3 towards the cathode 4 causes the cathode to release a further quantity of photoelectrons which increase the electron stream and 1 thus, in turn, increase the light emission by the luminophore 3. By analogy with retroaction in thermionic valve circuits, this process may be termedand is hereinafter referred to as-a retroactive light amplification. The process is repeated a number of times, causing constant increase of the photocurrent density and, consequently, of the light emission by the luminophore 3.

Normally this effect is probably in the nature of a geometrical progression with a finite limit. If thelimit, which is mainly dependent upon the photosensitivity of the cathode 4, the energy conversion efficiency of the luminophore 3, the translucency of these two layers, and the operating voltages, is made high enaough to give a sufficient gain per single stage of retroactive light amplification, a high energy conversion eiliciency. of the lamp combination may be obtained.

Furthermore, if conditions are suitable, a selfactivation may arise, independent of the initial irradiation of the photoemissive cathode 4 by the light source 5. If this occurs, the power energising the light source 5 may be reduced for example, as shown schematically by a variable resistor 52 after the cathodo-luminescent lamp has started, or the light source 5 may be switched o for example, by a switch 53. If some light is present from other sources, such as natural light (diffused daylight), or other nearby lamps, and this light reaches the photoemissive cathode 4 and thus lberates photoelectrons, an auxiliary light source may be dispensed with.

It is convenient to select a luminophore having a substantial phosphorescence for the surface 3. The phosphorescence need be of a short duration only as long as it is approximately equal to, or longer than the travelling time of the photoelectrons from the photoemissive surface 4 to the luminescent surface 3. A zinc sulphide activated with copper, or a mixture of zinc sulphide and zinc-cadmium sulphide is a suitable phosphor for this purpose.

It will be appreciated that the invention not only increases the specific electron current density and, consequently, the luminous flux from the luminophore but, in addition, may render the distribution of the electrons as supplied to the surface of the luminophore substanially uniform and may, thus, make the distribution of the light emission around the lamp centre, or the lamp axis, approximately symmetrical.

In a modification of the embodiment so far described with reference to Fig. l, two differently luminescing translucent luminophoric layers are provided, one behind the other; the luminophore 3 is adapted to emit light in the shorter wavelength part of the spectrum, particularly violet and ultraviolet light, whenexcited by the photo electrons. In this case, the layer 3 may be positioned on a carrier adapted to transmit readily short-wave light, for example on the interior wall of the envelope 2. The other luminophore 'l (Fig. 1) may be arranged on the exterior surface of the tube envelope 2 and is excited by the short-wave-light luminescence of the luminophore 3. At the same time, the photoemissive surface of the cathode 4 which, in this case, advantageously has its maximum spectral sensitivity in said short-wave region, is caused to liberate further photoelectrons under the shortamplification takes place.

wave-light emission partly cast back by the luminophore 3, so that again a retroactive light In this case, the photoemissive surface 4 may be a potassium layer. The short-wave-light-emitting luminophore may contain an ultraviolet luminescing compound of salts of one or more of the group I elements, such as rubidium chloride, or sodium chloride. or it may contain a cadmium chloride.

In the preferred embodiment shown in Fig. 3, a tubular glass envelope B contains a gas filling and is surrounded by a second envelope 9, the annular space between the walls of the envelope 8 and 9 constituting a closed chamber, or compartment, which contains a photoemissive cathode I0 and a conductive luminophore-anode ll, similar to the anode 3 in Fig. l. In this embodiment, however, the photoemissive surface Ill is arranged on part of the interior surface of the second envelope 9; and the luminophore Il is arranged as a translucent conductive layer on the opposite side, i. e. the radially outer surface of the envelope 8. Therefore, in this embodiment, the photoemissive surface surrounds the luminescent layer. Since here the area of the photoemissive surface I0 is larger than that of the luminescent layer Il, the density of photoelectrons effective per unit area of the luminophore is higher than that in the arrangement of Fig. l. In addition, the part of the envelope 3 carrying the photoemissive surface is provided with grooves, furrows or the like, as indicated in Fig. 3. such as to give the photoemissive surface a starlike section, in order to increase its effective photoemissive area. If desired, part 5B of the envelope 8 carrying the luminescent layer l I may be given a similar shape in order to increase its effective luminescent area. The photocathode lll and the conductive luminophore Il are connected to a suitable direct-current source l2. The envelope 8 may contain a rare gas or vapour or a mixture thereof at a low pressure, such as argon and a small quantity of mercury, and electrodes 49, 50 spaced apart lengthwise to provide for a positive column discharge between them upon energising as in a conventional elongated gaseous discharge tube. Suitable circuits for starting and operating elongated gaseous discharge tubes are well known. For example, a circuit including a ballast and starter switch may be employed such as illustrated in Fig. 10 on page 22, or in Fig. 42 on page 46 of the book by Charles L. Amiek, Fluorescent Lighting Manual, first edition, New York and London, 1942. If desired, also the interior surface of the envelope 8 may be coated with a luminescent material 55. The luminophore Il or the luminophores, and the photoemissive cathode I0 cover the respective envelopes only partly and constitute, for example, hemispheres or half-cylinders. Accordingly, an uncoated, clear part window I3 is left, through which the light can emerge. In this arrangement, the photo-emissive cathode I 0 need not be translucent, but may consist of a more compact layer.

In operation, the visible and near-visible radiation produced by the gas discharge within the envelope 8, upon energising the electrodes as referred to above, passes through the wall of the envelope 8 which, of course, must be sufciently translucent for the said radiation or its effective part, and excites the translucent luminophore Il, or the luminophores, to luminescence. The light emitted by the luminophore ll, or the luminophores, in turn, falls on the photosensitive layer l and liberates photoelectrons thereon, whereupon again a retroactive light ampliflcation takes places. Also some of the light of the gas discharge, especially that part which penetrates or is diffused by the translucent 1uminophore I I, or by the luminophores, directly iniluences the photo-emissive cathode I0. The mixed light produced by the gas discharge, and the luminescence light emitted from the luminophore II, emerge through the window I3, the luminescence light traversing the gas space. It may be 'arranged that the luminophore II is excited to luminescence simultaneously by photoelectrons from the photoemissive coating I8 and by those light components of the gas discharge which lie in the short-wave region and which are able to penetrate the envelope 8.

In order to increase the light output of the device, two (or more) auxiliary electrodes Il, I (Fig. 3) may be provided between the photoemissive surface I8 and the luminescent layer II,

which electrodes are sensitised for secondary.

electron emission. The electrodes Il, I5 may consist of perforated plates, or iine wire mesh grids of nickel, which are furnished with ay secshape of the envelopes 8 and 8. Alternatively, the

electrodes may be given any other desired shape, for example a polygonal or star-like cross section. The electrodes I4, I5 are supported and separated by spacers I6, I1, I8. As a secondary emitting substance, a layer of caesiated silver oxide may be provided on the electrodes Il and I5, or they may be made of a suitable metal alloy.

As before, the photoemissive layer l0 is connected to the negative pole of the source of potential I2, the positive pole of which is connected to the conductive luminescent layer II. The secondary-electron-emissive electrodes I4 and I5 are each on an intermediary potential which increases progressively in the direction of the conductive luminescent layer II which, therefore, constitutes the main anode of the system. The gaseous discharge system in the main envelope 8 may, of course, be energized from the same source I2 as the cathodo luminescence system.

In operation, in this modification of Fig. 3, the electron stream liberated from the photoemissive surface I0 is increased by secondary emission multiplication from the electrodes Il, I 5 before impinging on the luminescent layer I I. That part of the light, which is radiated by the luminescent layer II towards the photoemissive surface I0, falls partly on the latter and partly on the electrodes Il and I5. The cast-back light may cause the secondary-emissive electrodes Il and l5 to release quantities of photoelectrons which further increase the multiplied electron stream and thus increase the luminous output of the device. To this effect, the surfaces of the electrodes Il and I5 are preferably sensitized, in a known manner, so as to render them highly photo-electrically-emissive and secondary-electron-emissive.

In analternative arrangement, the gas discharge may take place in the annular space between the envelopes 8 and 9, and the photoelectric discharge within the envelope 8. A transparent photoemissive cathode is then arranged as a coating on the interior surface of the envelope 8, and the luminophoric layer is arranged opposite on a carrier, for example a glass cylinaccording to Fig. 3 and its modifications the possibiiity of employing a photoemissive cathode I8 consisting of a compact layer affords the advantage of a higher sensitivity and good light reflectivity. Furthermore, a combined light is produced, which fact permits a considerable extension of the variety of suitable luminescent materials and gas-fillings and accordingly a wide range of colours and shades and, also, the use of constituents hitherto considered unsuitable. The simultaneous excitation of the luminescent layer II by aetherial radiation and charged particles may also effect changes of the colour of luminescence of suitable phosphors, such as certain zinc silicate compounds, and cadmium tungstate activated with uranium; in some cases it may also improve their energy conversion efficiency (as further referred to hereinafter); and, in fact, it is possible to prepare luminophores which are sensitive to both kinds of exciting agents as, for example, a zinc sulphide luminophore containing two heavy metals, namely copper and gadolinium, which cause emission respectively ln the visible band and mainly in the ultra-violet range of the spectrum. In such a luminophore layer the upper portion is excited by the cathode rays to emit both kinds of radiation. The lower portion is not reached by the cathode rays and is excited by the ultra-violet radiation of the gadolinium in the upper portion.

In the embodiment shown in Fig. 4, again a gaseous discharge lamp l1 is associated with a cathodo luminescent lamp I8. However, two differently arranged tubular envelopes, one for each lamp, are provided in this case. The gas discharge lamp I1 is partly coated on its interior surface with a translucent luminophoric layer I8, leaving clear a window .20 which extends in a substantially straight band over the greater part of the length of the lamp envelope. The cathodo luminescent lamp I8 comprises a tubular envelope 2|, the interior surface of which is partly coated with a translucent luminophore 22, so as also to leave clear a straight window 23 extending over the greater part of the length of the envelope. Arrangedwithin the lamp I8 is furthermore a translucent cylinder 24, for instance of glass, which is coated on its exterior surface with a translucent photoemissive material 25 consitituting the photoemissive cathode of the lamp I8. The two lamps I1 and I8 are preferably arranged approximately parallel and close to each other with their windows 20 and 23 in opposition. II desired, secondary-emissive auxiliary electrodes similar to those (I4, I5) in Fig. 3, maybe provided, mounted on the carrier 24, and the respective electrical connections for energising the lamps I1 and I8 may be similar to those in Fig. 3. 26 and 21 are two light diffusing plates made, for example, of frosted glass. which bridge the gap between the lamps I1 and I8 such as to give the lamp combination the appearance of a unitary device.

In operation, some of the radiation emitted by the gas discharge in lamp I1, supplemented by some of the luminescent light emitted :by the luminophore I9 upon excitation by the gas discharge, passes through the windows 20, 23 and ex cites the photo-emissive cathode 25 of the cathodo luminescent lamp I8 whereupon the retroactive light amplification takes place.

In this embodiment, the two lamps I1 and I8 in generalv emit the greater part of the produced iight separately, and it is obvious that such a combination can be usefully employed, for example, for special light effects, colour correction, and the like.

The association of the cathodo luminescent lamp with a light-emitting gaseous discharge tube, such as shown in Fig. 3 or 4, is also advantageous from another aspect. If, for example, the gas discharge is a mercury discharge at low pressure, the ultra-violet energy can be transformed into visible light to a large extent by a suitable luminophore in contact with the discharge. However, part of the energy, say 25 to 35 per cent, is radiated in the infra-red region. A large portion of this otherwise ywasted energy can now be converted into electron energy, if the photo-emissive surface (I0 or 25) in the cathodo luminescent lamp is suitably energised. A caesium oxide layer is especially useful in this respect as a photoemissive material since it can be rendered sensitive in the region of the near infra-red as well as for visible light, this fact being particularly advantageous for effecting retroactive light amplification if the photo-emissive material is associated with a luminophore emitting predominantly visible and long-wave light.

In some cases it may be convenient not to specially sensitise the photoemissive surface, but to use as a cathode a plain metal cylinder or the like which may, for example, consist of zinc, nickel, or aluminum and which, preferably, is made light-reflective on its outer surface. Such a cathode also becomes photoemissive under irradiation by light, particularly by light of a short wave-length. Since, in general, the number of photo electrons liberated from a cathodesof this type is only comparatively small, the provision of secondary-electron-emissive auxiliary electrodes, such as I4, I5, in Fig. 3, will be particularly effective.

The improved cathodo-luminescent lamp, such as, for example, the lamp IB shown in Fig. 4, may be surrounded, wholly or partly, by a transparent liquid 28. The liquid 2B may be contained in a chamber or compartment, enclosed by the envelope 2| and a second glass envelope 29, which surrounds the envelope 2l Wholly or partly, and may be supported by annular support members positioned on the lamp bases. The liquid 28 may be an electrolyte of a good conductivity, for example, an aqueous solution of an alkaline metal or similar salt of a weak acid or alcohol or both. Instead of glass, any other suitable transparent material may be used for the second envelope 29, for example a plastic material; and instead of a liquid, a semi-fluid or solid substance may be used, such as a viscous electrolyte containing glycerine and alkali salts as constituents.

The translucent electrolyte 28 may be connected to ground, for example by means of an immersed electrode 30 of aluminium or copper. Thus, any direct wave radiation, which may be due, for instance, to discharge oscillations within the lamp and which may cause radio interfer- 8 ence, can be reduced to a low level, or can be substantially eliminated.

It is well-known that, in applying a metal screen such as usually employed for shielding purposes, it is necessary to choose the mesh size for a maximum reduction of interference and minimum absorption of light. The electrolyte screen 28 obviously oiers a great advantage in that respect.

In some cases it may be advisable, in addition to screening the lamp, to reduce or eliminate any interfering radiation from the supply line, and any power line feedback, by shielding the leadin wires, or using twisted pair leads, and by application of line lters according to known principles.

If the filling 28 is a liquid, particularly if it is a water electrolyte, it cools the lamp I8, and especially the luminophore 22, and thus performs the double function of cooling and substantially suppressing the direct radio-frequency radiation, without materially affecting the light emission from the lamp I8. The grounded electrolyte screen 28 affords the further advantage that the discharge within the lamp is protected against external inuence.

Fig. 5 shows an embodiment of the invention adapted to operate on alternating current. The lamp comprises a tubular glass envelope 3l which has double walls 32, 33 approximately parallel to each other, somewhat similar to the envelope shown in Fig. 1. The substantially annular space between the walls 32, 33 is relatively narrow, as in a Dewar vacuum flask, the walls thus forming two co-axial cylinders. Both the elongated interior surfaces of the lamp envelope 3| are coated with a luminescent material 34, 35, for example, a phosphorescent zinc silicate, or zinccadmium sulphide. The use of a luminescent material showing phosphorescence is advantageous in securing an effective light feedback as stated above, and for other reasons given hereinafter in the description of the operation. The luminophores 34, 35 are rendered conductive ln any suitable manner. The luminophore 35 on the inner wall 33 of the double-walled lamp envelope inner luminophore may be backed by a solid metal plate which is preferably highly light reflecting. The luminophore 34 on the outer wall 32 of the lamp envelope outer luminophore is translucent and has a translucent conductive backing which may consist of a thin silver deposit or a metal grid. In order to secure a substantially uniform distribution of potential over the thin silver layer or metal grid, one or more strips of a thicker metal layer may be provided, extending over the length ol' the tube 3l, and also in the form of rings near the tube ends, and electrically connected to the said thin silver deposit or metal grid. The two luminophores 36, 35 are preferably made highly secondary-electron-emissive. This may be done, for example, by mixing the luminescent material with small amounts of barium, calcium, or

caesium or by depositing, on the luminescent surface, a thin conductive electron-permeable layer of a metal having a low work function such as one of the above-named metals. The conductive parts of the two luminescent layers 34, 35 are connected respectively to the terminals 36, 31 of the secondary winding of a transformer 38, the primary winding 39 of which is connected to a. source of alternating current of audio frequency.

A third electrode 49 is provided. This electrode may be constituted by a grid o! une wire mesh made, for example, of nickel and bent to form a hollow cylindrical body, which is located in the substantially annular space between the two luminophores 34, 35 and extends approximately parallel 'to the axis of the cylindrical doublewalled lamp envelope 3|. This grid electrode 4|! is furnished with a photo-electrically-emissive surface, which is preferably also secondary-electron-emissive, the active material either filling the mesh, or, preferably, surrounding the gauze wires. The grid electrode 40, therefore, constitutes an electron-emissive, electron-permeable, and light-permeable hollow body of a large diameter or cross-section and, accordingly, with a large electron-emissive surface near to the luminophore. Consequently, an effective interaction can take place between the lumincphores 34, 35 and thegrid electrode 40. The grid electrode 40 may be connected to a. centre tapping 4I of the transformer secondary winding-and may also be connected to ground.

An auxiliary light source 54, for example of the cathode-glow type, maybe provided, for example positioned within the open hollow space surrounded by the double-walled envelope 3l, similarly to the light source 5 in Fig. l, to cause photo-electrons to be liberated within the lamp and, thus, to provide the initial current. In this case the conducting backing of the inner luminovphore 35 should, of course, be translucent, or the latter should be non-conductive and transparent as hereinafter referred to. Frequently, an auxiliary light source may be dispensed with since, owing to the low work function of the large activated surface of the grid electrode 40, usually a certain amount ofthermionic electron emission from the surface takes place, even at room temperatures, which, under suitable conditions, may be sufficient to initiate the discharge.

In operation, the photoelectrons released from the grid electrode 40 are accelerated alternately towards the inner and outer lumincphores 34 and 35, under the influence of the alternating audio-frequency electric field set up between the two lumincphores, and they excite the luminophores to emit light.

Under suitable conditions, a number of reciprocal actions and effects will then occur within the discharge space of the electric lamp, which may beV listed as follows:

( 1) Increasing of the luminescence by increasing the initial current due to (a) acting and reacting of the luminophores 34, 35 and the activated grid electrode 40 upon each other retractive light amplication;

(b) phosphorescence (afterglow) of the lumincphores 34, 35, which phosphorescence causes an additional photoemission from the grid electrode 40,and reduces flickering by steadying the light output of the lamp;

(c) secondary electron multiplication, by the grid electrode 40, of photoelectrons incident thereon from the lumincphores 34, 35 (if the latter are prepared so as to be capable of emitting photoelectrons besides light).

(2) Increasing of the luminescence due to mutual photo-excitation by the short-wave components of the light emitted from theA luminophores 34 and 35.

In a modification of the embodiment shown in Fig. 5 as so far described, the outer luminophore 34 is non-conductive. This simplifies manufacture and avoids loss of light by absorption which, otherwise, would take place in the conductive metallic nlm or grid. A non-activated metal grid cylinder 42 of comparatively wide mesh is arranged close to the outer luminophore 34,

the luminophore from becoming charged negatively by the impacting primary electrons. In general, the luminophore 34 will then take up an equilibrium potential value at a positive potential. A high positive potential of the non-conductive luminophore 34 is desirable also for attracting photo-electrons, released from the activated grid electrode 40 during the negative phase and the phase change of the alternating field. The release of photoelectrons from the activated grid electrode 40 is caused, during these phases, by phosphorescence of the luminophore.

'Ihis embodiment may be further modified by making also the inner luminophore 35 non-conductive, and providing a further non-activated wide-mesh grid cylinder 43 close to it. The two wide-mesh grid cylinders 42 and 43 are connected to the terminals 31 and 36 of the centre-tapped transformer winding. The activated grid electrode 40 is located between them and again connected symmetrically to the transformer. If desired, the two wide-mesh :grid electrodes 42 and 43 may also be activated. 'Ihe wide-mesh grid electrodes 42, 43 may be connected asymmetrically to the transformer secondary. Then, one of the two wide-mesh grid electrodes will be at a higher voltage relative to the activated grid electrode 40 than the other.

According to another modification of the embodiment shown in Fig. 5, two (or more) additional grid cylinders 44 and 45 which are photoelectrically-emissive or secondary-electron-emissive, oi' capable of both kinds of emission, may be provided between the wide-mesh grid cylinders 42, 43 and the centre grid 40,. which additional activated grid cylinders are connected respectively to tappings 45 and 41 on the two sides of the transformer centre tapping 4| which is connected to the centre grid 40. y

Here, substantially the same effects and interactions as in the rst and second embodiments take place. But, obviously, the secondary electron multiplication by the grid cylinders 44 and 45 contributes more effectively towards increasing the initial electron stream. To the list of reciprocal actions and effects, in this case the following one may be added:

(1) (d) mutual secondary electron multiplication, by the two activated grid cylinders 44 and 45 and the wide-mesh grid cylinders 42 and 43 (if activated), of their photoelectron emission.

The last described embodiment may be modified by arranging as a third luminophore a luminescent screen, preferably of cylindrical shape.' between the'two photoemissive and secondary emissive grid cylinders 44 and 45. The third luminophore is light-permeable as well as electron-permeable. It may, therefore, consist of a perforated supporting member, for example, of cylindrical shape, made of glass or mica, which is coated with translucent layers of luminescent material on both its outer and inner surfaces. Alternatively, a metallic or non-metallic grid cylinder may be provided which is furnished with the luminescent material, preferably surrounding the gauze wires. The luminescent material coated on said supporting member or grid cylinder may be of a kind emitting predominantly visible or invisible, for example, shortwave light. The third luminophore may take the place of the centre-grid electrode 40.

Though the lamp envelopes described above have a circular shape, they may also be given another shape. For example, the lamp envelopes may have an oval cross-section. The same applies of course to the grid electrodes.

The cathode-luminescent lamp according to this invention may be manufactured by employing two separate cylindrical envelopes of different diameter, similar to the walls 32, 33 in Fig. 5, which are combined to form the lamp by arranging them coaxially. The outer cylindrical envelope (corresponding to 32) may be coated with a luminescent material on its interior surface, and the inner cylindrical envelope (corresponding to `33) may be coated with a luminescent material on its exterior surface, and may serve as a support for the grid electrodes which are to be mounted on it, for example by spacers such as I6, I1, I8 in Fig. 3. The inner cylindrical envelope is preferably evacuated and sealed off before inserting it into the outer envelope. The substantially annular enclosure formed between the walls of the two cylinders is then evacuated and provides the space within which the electron discharge, substantially perpendicular tothe tube axis, takes place. The "inner cylinder may be supported at the ends only. For example, a neck, formed by a diminishing cross-section at one end of the outer envelope, may be provided, into which the inner" envelope is fitted. This end may be capped, for example, with a standard type of lamp cap which, when the electrical connections are arranged at the other end of the lamp, need not have any conductive leads into the discharge space and, in fact, need not have any conductive parts at all, being a dummy cap. i

It will therefore be apparent that this invention provides the following advantages:

Both the light-emitting layers and the elctronemitting surfaces present large effective areas; accordingly, their respective total emissions are large, and not unduly high specific loads are required in order to secure a high efciency of the lamp.

An effective interaction between the lightemitting luminescent layers and between the latter and the electron-emitting surfaces can take place, owing to their close proximity.

The small volume of the annular discharge space within the tube facilitates pumping.

When the tube is highly evacuated, any undesired ionization of residual gas by collision is negligible, even when, owing to the release of small quantities of gas after the tube has been sealed, the pressure rises to some extent; this considerably increases the normal operating life of the tube.

It will be apparent from the foregoing description that the claims are to be interpreted in the light of the following definitions:

Cold-cathode type-having a cathode that does not employ any electrical circuit which supplies current for the sole purpose of heating the cathode.

Evacuated envelopehighly evacuated or containing only traces of gas.

Electrodean element within the envelope, or a layer on a carrier or on the inner surface of the wall of the envelope not necessarily having a iolid electrical connection to an external circu Luminescentcapable of luminescence (not necessarily exclusively) -by impact of electrons.

Minimum linear dimensionof a cylindrical luminescent layer or of a cylindrical photo-electrically-emissive surface is its circumference or its length, according to which is the smaller, and

in the limiting case of a sphere it will be the circumference of a great circle.

"Convex and concave-having a section, in either only one or both of two planes at right angles to each other, which is respectively dished or bulged, e. g. tubes or spheres.

Approximately cylindrical-includes polygonal and longitudinally corrugated with equivalent points on the corrugations on a polygonal or cylindrical surface.

Gas discharge-discharge in rare gas or vapour or ina mixture of rare gas and vapour.

I claim:

1. An electric discharge lamp of the coldcathode type and including an evacuated lighttransmitting envelope containing an electrode having a surface adapted to emit photo-electrons and a translucent carrier electrode spaced therefrom bearing a luminescent layer in the path of said photo-electrons and capable of emitting light-radiation when excited thereby, wherein the said surface and layer are atleast approximately parallel to each other and spaced apart at a distance shorter than the minimum linear dimension of either of them, and an auxiliary light source in position to illuminate said first-mentioned surface to cause the initial emission of photo-electrons therefrom, whereby said electrically-emissive surface is exposed to at least part of the light radiation emitted from the side of said luminescent layer that faces said photoelectrically emissive surface, and said light radiation causes the release from said photo-electrically-emissive surface of photo-electrons which travel oppositely to the said light radiation and excite said luminescent layer, said lamp also including an auxiliary luminescent element capable of being excited by light of relatively shortwave-length emitted from the other side of said luminescent layer, and light radiation from said auxiliary element being emitted by the lamp.

2. An evacuated electric discharge lamp comprising an evacuated light-transmitting envelope, an electrode therein having its surface adapted to emit photo-electrons, another electrode therein, said other electrode being of translucent conductive material substantially parallel to said firstmentioned electrode and spaced therefrom a distance shorter than the minimum length of either, a coating of luminescent material on said translucent electrode and in the path between said first-mentioned electrode and said translucent electrode, and an auxiliary light source for illuminating said first-mentioned electrode to cause it to emit photoelectrons, whereby said electrons from the first-mentioned electrode flow to said other electrode, exciting the luminescent material to emit light part of which passes outward from said discharge through the translucent electrode and light-transmitting envelope and part of which passes backward toward the photo-electrons, and electrical connections for establishing operating potentials on the electrodes, the disposition of said surfaces being such that.- when said connections are suitably energized, the surface adapted to emit photoelectrons and the luminescent surface will cooperate to eilect a retroactive light amplification, while the secondary-electron-emissive surface will eil'ect electron multiplication.

4. The electric discharge lamp of claim 2, an electrode capable of secondary electron-emission and between the coating of luminescent material and the surface adapted to emit photo-electrons, and electrical connections for establishing operating potentials on the electrodes, the disposition of said surfaces being such that, when said connections are suitably energized, the surface adapted to emit photo-electrons and the luminescent surface will cooperate to effect a retroactive light amplification, while the secondaryelectron-emissive surface will effect electron multiplication, at least one of said electrodes being provided with a surface adapted to emit photo-electrons which is also secondary-electronemissive.

5. The electric discharge lamp of claim 2, wherein said electrodes are disposed in such relationship that, when the lamp is suitably energized, said luminescent material is excited by photo-electrons emitted from said surface adapted to emit photo-electrons and that said surface is at least partly exposed to the light radiation emitted by said luminescent material, the combination being such that said light radiation causes the release of photo-electrons from said surface, said lamp also including an auxiliary gas discharge device positioned to illuminate said surface and so arranged that part of the light which it produces emerges from the lamp.

6. The electric discharge lamp of claim 2 wherein said electrodes are disposed in such relationship that, when the lamp is suitably energized, said luminescent material is excited by photo-electrons emitted from said surface adapted to emit photo-electrons and that said surface is at least partly exposed to the light radiation emitted by said luminescent material, the combination being such that said light radiation causes the release of photo-electrons from said surface, said lamp also including an auxiliary gas discharge device provided with a luminescent coating, said auxiliary discharge device being positioned to illuminate said surface and so arranged that part of the light which it produces emerges from the lamp.

7. The electric discharge lamp of claim 2 in which the evacuated light-transmitting envelope is of annular section and the auxiliary light source is a gas discharge device surrounded by said envelope and so arranged that part of the light which it produces emerges through said envelope, said gas discharge device containing an electrode having a photo-electrically-emissive surface and an electrode having a translucent luminescent surface which surfaces only partly surround said gas discharge device, said photoelectrically-emissive surface being radially outside said luminescent surface. said electrodes f being disposed in such relationship that, when 14 the lamp is suitably energized, said luminescent surface is excited by photo-electrons emitted from said photo-electrically-emissive surface and said photo-electrically-emissive surface is at least partly exposed to the light radiation emitted by said luminescent surface, the combination being such that said light/radiation causes the release of photo-electrons from said photo-electrically-emissive surface.

8. 'I'he electric discharge lamp ofclaim 2, in which the auxiliary light source is a gas discharge device having an envelope of annular section, the core of which is occupied by an evacuated chamber containing an electrode having a photo-electrically-emissive surface and an electrode having a luminescent surface, wherein said electrodes are disposed in such relationship that, when the lamp is suitably energized, said luminescent surface 'is excited by photo-electrons emitted from said photo-electrically-emissive surface and that said photo-electrically-emissive surface is at least partly exposed to the light radiation emitted by said luminescent surface, the combination being such that said light-radiation causes the release of photo-electrons from said photo-electrically-emissive surface and part of the light produced from said auxiliary device emerges from the lamp.

9. The lamp of claim 8, wherein said photoelectrically-emissive surface is translucent and radially outside said luminescent surface.

10. The electric discharge lamp of claim 2, wherein said electrodes are disposed in such relationship that, when the lamp is suitably energized, said luminescent material is excited by photo-electrons emitted from said surface adapted to emit photo-electrons and that said surface is at least partly exposed to the light radiation emitted by said luminescent material, the combination -being such that said light radiation causes the release of photo-electrons from said surface, and in which the auxiliary light source is a gas discharge device having an envelope placed side by side with said evacuated envelope, and said electrode surfacesbeing disposed in the part of the interior of said evacuated envelope that is remote from said auxiliary discharge device.

l1. The electric discharge lamp of claim 2 adapted to operate on alternating current and including an additional electrode having a coating of luminescent material, whereby there are provided two electrodes having coatings of luminescent material, and electrical connections for establishing operating potentials on the electrodes such that, when said connections are suitably energized, an alternating electric field is established which will direct alternately onto said two electrodes electrons derived from the surface adapted to emit photo-electrons.

12. The combination of claim 11, in which the electrodes having coatings of luminescent material are also secondary-electron-emissive.

13. The electric discharge lamp of claim 2, an additional electrode having a coating of luminescent material, and in which the surface adapted to emit photo-electrons is a grid electrode disposed between the other two electrodes.

14. A lamp as claimed in claim 13, wherein said photo-electrically-emissive grid electrode is also secondary-electron-emissive.

15. The lamp of claim 14, wherein said photoelectrically-emissive grid electrode is also secondary-electron-emissive.

16. The electric discharge lamp of claim 2.

and an additional electrode having a coating of luminescent material and in opposed relationship to said other electrode having such a coating, two auxiliary grid electrodes each disposed adjacent one of the electrodes having the coating, and in which the electrode adapted to emit photo-electrons is a grid electrode between said two auxiliary grid electrodes.

17. The electric discharge lamp of claim 2, the envelope of which is at least partly enclosed by a. translucent solid electrolyte which serves as a grounded screen.

18. The combination of claim 1, in which the photoelectric surface of the first-mentioned electrode is at least one-quarter of the area of the luminescent coating.

19. The combination of claim 1, in which the photoelectric surface of the iirst electrode is convex while the luminescent layer is concave.

HANS RUDOLF LEVY.

REFERENCESl CITED The following references are of record in the 111e of this patent:

Number Number 20 366,883 812,946

16 UNITED STATES PATENTS Name Date Dodds Aug. 5, 1919 Miller Aug. 13, 1929 Waldschmidt Apr. 28, 1936 Schaffernicht Sept. 14, 1937 Farnsworth Feb. 8, 1938 Massa Oct. 31, 1939 Germer Sept. 3, 1940 Biele Nov..19, 1940 Leverenz (a) Sept. 22, 1942 Leverenz (b) Oct. 13, 1942 Leverenz (d) Mar. 16, 1943 Leverenz (c) June 29, 1943 Gessel Apr. 11, 1944 Leverenz (e) July 16, 1946 FOREIGN PATENTS Country Date Great Britain Feb. 11, 1932 France Feb. 15, 193'? Great Britain Nov. 9 19" 

